1. Field of the Invention
The present invention relates to an air-intake system of a multi-cylinder engine mounted in a small watercraft such as a personal watercraft.
2. Description of the Related Art
An engine body mainly comprises a cylinder head on the upper side, a cylinder block located below the cylinder head, and a crankcase located below the cylinder block. The inline multi-cylinder engine is substantially flask-shaped, as seen in a cross-sectional view of the engine when sectioned in the direction perpendicular to the longitudinal direction of the crankshaft. The cylinder block has a width smaller than that of the crankcase.
In general, an air-intake system of the multi-cylinder engine mounted in the small watercraft is configured to have an ambient-air inlet, an air cleaner for cleaning ambient air taken in from outside through the inlet, a throttle body for adjusting an air-intake amount, an air-intake box having a certain volume to temporarily store the air (sometimes omitted), and an air-intake pipe serving as a passage through which air is delivered into an air-intake port of the cylinder head. In this air-intake system, these components are connected in this order.
When the air-intake box is contained in a relatively limited inner space of the small watercraft, it is typically located laterally of the cylinder block having a small width.
Engine power is influenced by various factors. One factor is volumetric efficiency. As used herein, the volumetric efficiency refers to a ratio of the volume of air actually drawn into the cylinder to the displacement of the engine. The engine power is increased by improving the volumetric efficiency, i.e., by increasing the amount of air actually drawn into the cylinder during an operation of the engine (hereinafter referred to as an “air-intake amount”.)
Especially in the engine mounted in the small watercraft which is required to have a small size and a light weight, in order to generate high power with small displacement of the engine, it is desired that the power be increased by increasing the volumetric efficiency of the air to be drawn into the cylinder.
The volumetric efficiency (air-intake ability) can be improved by designing a structure of the air intake-pipe or the like through which air is drawn into the cylinder of the engine in view of air-intake inertia and pulsation effects. That is, if the engine speed required to generate high power is determined, an optimum length of the air-intake pipe may be determined.
Specifically, in the case of the engine required to generate high power during relatively high-speed rotation, the length of the air-intake pipe is preferably set relatively short. On the other hand, in the case of the engine required to generate high power during relatively low-speed rotation, the length of the air-intake pipe is preferably set relatively long. In this manner, the air-intake ability can be improved by designing the air-intake pipe or the like according to the optimum wavelength of the pulsation flow determined by the engine speed.
Commonly, a longer air-intake pipe is used in engines mounted in small watercraft required to generate high power during relatively low-speed rotation. However, in multi-cylinder engines mounted in small watercraft, the air-intake box is located laterally of the cylinder block and close to the air-intake port, therefore the air-intake pipe connecting the air-intake ports to the air-intake box is difficult to extend.